Phenotyping of animal models and man for their xenobiotic metabolizing capabilities has, in recent years, been undertaken in the interests of predicting carcinogenic susceptibilities. The metabolism of exogenous agents is known to be genetically dependent, and selective in vivo xenobiotic metabolic routes appear to be accessible to evaluation by the use of nontoxic doses of certain drugs. The determination of the rate of selective enzymatic modifications of test agents may thus provide a suggestion as to how susceptible an individual may be to the oncogenic potential of carcinogens activated by similar metabolic routes. Several agents including debrisoquine, S-mephenytoin, S-carboxymethyl-L-cysteine, and sulfamethazine have been shown to be metabolized by enzymatic routes governed by separate genetic loci. Thus, these agents will be used to phenotype nonhuman primates on the basis of their abilities to metabolize these compounds. Since the primate colony from which these monkeys will be sampled has been and is presently involved in ongoing chemical carcinogenesis experiments, the results of the metabolic phenotyping can be compared to the susceptibilities of the monkeys to carcinogenesis.